1. Field of the Invention
The present invention relates to an internal combustion engine and more specifically to an arrangement which permits the compression ratio of the engine to be selectively controlled.
2. Description of the Prior Art
FIG. 1 shows a prior art arrangement disclosed in Japanese Utility Model Pre-Publication No. 58-25637. This arrangement includes an outer piston 10, an inner piston 12 reciprocatively received within the outer one in a manner to define a first variable volume chamber 14, a piston pin 16 which extends through the inner and outer pistons 10, 12, and a con-rod 18 which operatively interconnects the piston pin 16 with a crankshaft of the engine (not shown). An annular retainer 20 is threadedly received in the lower portion of the outer piston 10. This member is formed with a horizontally extending flange portion 21 which cooperates with a step formed in the bottom of the inner piston 12 to define an annular shaped variable volume chamber 22.
A source of hydraulic fluid under pressure generally denoted by the numeral 24 includes a sump or oil pan 26, a pump 28 which inducts oil from the pan, and a control circuit 30 which is responsive to a plurality of sensors 32 and which controls the operation of the pump 28. In the above mentioned document the sensors 32 are disclosed as being ones which sense the driving condition parameters.
The output of the pump 28 is supplied to the first variable volume chamber 14 via a first passage 34 which is bored or similarly formed in the con-rod 18, a second passage arrangement 36 formed in the piston pin 16 and a third passage 38 which extends through the inner piston 12 to the first variable volume chamber 14. A one-way valve 40 is disposed at the downstream end of the third passage 38 and arranged to prevent the back flow of hydraulic fluid which passes therethrough.
A control valve arrangement is disposed in a bore formed in the inner piston 12. This valve (as shown in FIG. 2) includes a spool 42 which is biased in one direction by a spring 44. This valve is arranged to be responsive to the pressure prevailing in the first, second and third passages 34, 36, 38 in a manner to control the amount of hydraulic fluid which is permitted to drain from the first variable volume chamber 14 via a fourth passage 46 which extends through the inner piston 14. The downstream end of a drain passage 48 which leads from the bore in which the spool 42 is disposed, is arranged to open into the inner periphery of the inner piston 12 as shown, to permit the hydraulic fluid to precipitate down toward the engine crankshaft and the oil pan 26 of the engine. A one-way valve 50 is disposed in the upstream end of the fourth passage 46 and arranged to prevent the return of any fluid which has flow out of the first variable volume chamber 14 into the fourth passage.
When the pressure discharge by the pump 28 increases, the spool 42 of the control valve is biased against the spring 44 in a manner which tends to prevent the flow of hydraulic fluid through the fourth passage 46 and thus ensure that the pressure in the first variable volume chamber 14 reaches that prevailing in the first, second and third passages 34, 36, 38. As will be appreciated, as the pressure in the first variable volume chamber 14 increases the outer piston 10 is biased to rise up away from the inner one 12 and in a direction which increases the compression ratio of the engine.
On the other hand, when the pressure discharge of the pump 28 lowers, the spool 42 of the control valve tends to move to the left as seen in the drawings and thus tend to open the fourth passage 46 in a manner which permits the hydraulic fluid which has been supplied into the first variable volume chamber 14 to be drained into the second annular variable volume chamber 22.
By controlling the level of the pressure discharged by the pump 28, the degree by which the outer piston 10 is displaced from the inner one can be controlled and thus permit the compression ratio of the engine to be controlled.
This form of compression control is highly advantageous in that, at low engine speeds a higher compression provides good engine response and acceleration while at higher engine speeds a lower compression ratio permit the engine speed to be increased without the fear of engine knock and/or in the worst case severe engine damage. The particular type of control also lends itself advantageously to use in Diesel engines which inherently have a high compression ratio. With Diesel engines the high compression ratio leads, under certain modes of engine operation, to the situation wherein the friction loss causes a power output reduction.
However, the arrangement disclosed above has suffered from a number of drawbacks which tends to inhibit practical application.
The first of these comes in that, during low compression operation wherein the first or upper variable volume chamber 14 is drained and the engine is operating at high speeds and a large amount of fuel is being combusted, the heat generated by the combustion causes the oil retained in the upper section of the third passage 38 in which the one-way valve 40 is disposed, to undergo degradation after prolonged exposure, and induce the formation of gummy tar residues and deposits which inhibit proper operation.
A second drawback comes in that a special pump must be provided. Viz., the output of the normal engine oil pump cannot be used as the output thereof is low at low engine speeds and cannot provide the required pressure level.
A yet further drawback occurs when it is required to reduce the compression ratio toward a lower value and it is necessary to drain the first variable volume chamber 14. During this operation the oil from the first variable volume chamber 14 is transferred to the second annular one 22 via the fourth passage 46. However, the cross-sectional area and volume of the second variable volume chamber 22 is less than the first 14. Thus, as the amount of fluid which must be transferred is greater than can be received in the lower chamber 22. During this mode the provision of seal elements 52, 54 on the inner piston 12 prevents leakage from the lower chamber 22. Accordingly the problem that the hydraulic fluid cannot be readily removed from the upper chamber 12 occurs. This tends to deteriorate the high to low compression transition response characteristics of the device.